Hydrofoil for Pontoon Boat

[SlowCruise]

I am looking for a way to speed up my pontoon boat a bit. In searching all of these threads on adding third pontoons, lifting strakes, etc, I came across this fascinating idea I will quote below...

In researching these, it seems I can put a foil between the pontoons at around the center of gravity, and get some major lift. Making my big pontoon come up on plane to some extent and bring more speed and fuel efficiency.

Id have tried to contact this person through email, and he may still get back, but I wonder if any of the rest of you have thoughts on how I should go about fabricating a hydrofoil for my pontoon.

I have access to a sheet metal shop with a lot of different equipment and capabilities. My brother in law is really good with this stuff and he can basically build whatever I need...but I don't exactly know what I NEED!

Boat Info:
2001 Crestliner Pontoon
60hp 4 stroke Mercury
2 pontoons 23" each
Dry weight 2000 lbs

My questions are:
What size
What shape
Where to position (ie how do I locate the center of gravity on my boat)

Any ideas?

Thanks in advance...

Originally Posted by u4ea32

Looks like a simple way to improve high speed performance.

A more efficient and high tech approach would be to use one or two horizontal foils spanning the pontoons.

The fundamentals: Planing speed is all about lift to drag. The higher aspect ratio the foil, the better the lift to drag (more lift for a given amount of drag). A planing bottom is effectively a lifting foil, but all lift is created by pressure on the bottom, no lift created by suction on the top.

A very high performance soaring airplane (glider) may have aspect ratios in the 30:1 range. Birds with great soaring ability (e.g., also with excellent lift to drag ratios) have aspect ratios in the 3:1 to 5:1 range.

Typical planning boats have wide hulls that have short immersed length when planing: perhaps a 1:1 aspect ratio (width to length). Stepped bottoms work because they greatly reduce the length of the wetted bottom, while leaving the width the same: perhaps a 2:1 aspect ratio. Lifting strakes, while they certainly provide lift, have an aspect ration of about 1:50, so have very little lift for the amount of drag produced. Lift, yes, but lots of drag.

Some builders of multihulls have had great success with adding a hydrofoil that spans the gap between the hulls. Such foils are intended not to lift the boat clear of the water, but to simply reduce the effective displacement of the boat that is carried by the hulls. Therefore, such foils do not have to be large, and they do not need to have any controlled surfaces. Documented results of adding foils to existing power catamarans include a 40% reduction in power required for a given speed. Typically, one puts a single foil between the hulls near the center of gravity. Simple, efficient. There exist

Look up articles on "foil supported catamarans" or hysucat, also the Corsair Foiler 2200.

[Doug Lord]

You might get the book: "Hydrofoils Design Build Fly" by Ray Vellinga. It is the bible for hydrofoil experimenters.You can probably find it on Amazon or PM me for further info. You could also contact Ray-probably after you go thru the book pretty thoroughly since he has built many hydrofoil powerboats.
Keep in mind that what you're asking for requires a good deal of engineering just to determine the feasibility of the project then it has to be very carefully looked at structurally-preferably by an engineer. Good Luck!

[PAR]

Again, SlowCruise, you can't add weight, drag and screw up the flow to the prop, then expect anything else, except slowing your boat down. You can't get something for nothing. All planing craft are power verses weight and hydrodynamic efficiency. You know your hydrodynamic efficiency is on the crappy side of things and you weight is about average, so you need a bigger engine. It's not an especially difficult set of variables to consider. Wings and extra pontoons might (if well placed and shaped) change portions of the ride quality possably, but you will slow down, not speed up.

[messabout]

Par is telling it like it is.

Please do not be insulted if I suggest that you are seeking to make a silk purse from a sows ear. This is not to insult your boat but to address reality.

Boats with round pontoons are more efficient at very low speeds because there is less wetted surface for a given displacement. For speed we have to forego the very low speed efficiency for more dynamic forces that are involved in lift. For speed, a flat or rectangular section is decidedly superior to round ones. More economical to operate at speed too.

There was a Florida outfit who made pontoon boats with rectangular, near flat bottomed sections. They were more efficient at speed, required less power for a given speed and weight. Alas I think that they might have gone under. Why? Everybody knows that a pontoon boat must have round sections as a result of divine decree. Wrong!

Consider that you might weld on some channel sections, the same width as the round sections, to the bottom of your cylindrical pontoons. Now you have a flat running surface. By all odds it will go faster with the 60 HP that you have.......unless you have it overloaded with all the weighty creature comforts that we all appreciate. This would be a simple, straightforeward improvement in speed potential. It still may not live up to your hopes and desires. If it were my boat I'd do it in a heartbeat. Simple, cheap,not likely to diminish present performance. One caveat. At the front end of the channel there should be a sled like transition from rectangular to round. That is not a challenging layout feature. Close the rear end of the channels too. The boat will float higher in the water now. You might have to lower the engine a tad, maybe not.

[SlowCruise]

I'm a little murky on your proposed layout.

What if I just created lifting strakes on both sides of each tube to achieve this flattening effect? If that what you are describing? (This would allow me to use the same trailer and not have to modify it.) I'll attach some pics so you might comment on the size only. I understand that you would have me seal them up and enclose the end there. I would definitely benefit from the added floatation.

And then rounding those into the tube and upward at the front? Or how exactly would I do the front rounding "sled" thing?

[Ad Hoc]

Originally Posted by SlowCruise

What if I just created lifting strakes on both sides of each tube to achieve this flattening effect?

Those, pictured, are not lifting strakes, they are spray rails.

On a hard chine boat, it would be a chine to separate the flow to assist in change from hydrostatic to hydrodynamic region.

In a nut shell, to get lift you need 2 things, 1) area and 2) speed. You have neither to achieve your objective. All you're doing is adding drag. Those spray rails shall never be above the running DWL as a spray rail/chine should be, thus just adding drag.

You should re-read PAR's comment again.

Dont break what is not broken!...if you want to go faster, you need a totally different hull form.

[Boston]

I've always been curious about this one. What if he's talking about displacement speeds tho. would the foil offer enough lift at lower speeds to reduce the wetted surface area and offer less drag overall. At say 10 knots, OK at 2 or 3 it might reduce efficiency but at say 6~10 it might improve it.

I realize he specifically quotes on planing forms but if it was just a displacement hull wouldn't a foil tuned to some specific speed range offer enough lift of offset its drag cost.

The arrangement of a straight foil between the hulls just seems to good to not work. Better yet two. One for and one aft, so there's no delicate balance to screw up.

[MechaNik]

I have looked at the benefit of foils on displacement craft before. It seemed that with a foil L/D ratio of 15:1 the "weight savings" effect on the displacement hull was still equal, only now with the complexity of a foil.

[Boston]

no appreciable gain in performance eh. Ok I guess that answers that

was there any savings in the structure of the boat. IE did the fact that you now had a connecting rod low and high equal more of a truss system to suspend the pod between the hulls making for an overall lighter support structure.

one big beam spanning x is always heavier than a truss. Could the foil be used in a truss system to help save some weight thus making its existence a worthwhile component of the system

I've also wondered about pitch dampening with a foil or foils. Would the lack of vertical motion translate to less power lost to other than forward motion ?

[Doug Lord]

Originally Posted by Boston

I've always been curious about this one. What if he's talking about displacement speeds tho. would the foil offer enough lift at lower speeds to reduce the wetted surface area and offer less drag overall. At say 10 knots, OK at 2 or 3 it might reduce efficiency but at say 6~10 it might improve it.

I realize he specifically quotes on planing forms but if it was just a displacement hull wouldn't a foil tuned to some specific speed range offer enough lift of offset its drag cost.

The arrangement of a straight foil between the hulls just seems to good to not work. Better yet two. One for and one aft, so there's no delicate balance to screw up.

=============================
Boston, here is a paper on Hysucat development where lifting hydrofoils were applied to planing and semi-planing catamarans with significantly good results:

[Doug Lord]

I wonder if this is any good? http://www.pontoonwaterglide.com/index.asp

[Ad Hoc]

Originally Posted by Boston

I've always been curious about this one.....At say 10 knots, OK at 2 or 3 it might reduce efficiency but at say 6~10 it might improve it.

As always, it is the application of the theory that dooms the “nice idea”.

Looking at those hulls, the distance between them is roughly, guessing 1.50m

Lets say you have a foil that spans, and you wish to get max lift possible, but min drag…so the typical 15:1 is that of say a simple section to make a NACA 0012.

If you go for broke and make the chord say 500mm, at 10knots you’ll get around 450kg of lift, great. So at 15:1 drag is about 30kg…and frictional about 10kg…great…all looking good so far.

So, let’s look at the structure.

On a 1.50m span lifting 450kg, using a suitable FoS of taking fatigue into account, for longevity, not the quick 5min wow factor….this gives you a modulus of 552cm^3.

A naca 12 section with a chord of 500mm, mean max depth of section is around 60mm, not much. Even being very “optimistic”, trying to get simple thickness of skins with a say 350mm section at say 25mm centres….you aint gonna do it! There is insufficient depth and/or strength in the aluminium (even if the skins were 25mm thick, ie solid ally, you only get around 280cm^3). Which means going to exotic materials for the strength, since you need a material property that has a high tolerance to fatigue and high fatigue limit. ….which means very expensive, more than the cost of you boat!

With boats, it is either hydrodynamic or structural reasons why most of the “that’s a good idea” fail, sadly.

[Boston]

this might be a completely stupid question but the drag effects the power needed to generate the lift. But the lift still lifts and reduces the immersed area of the hull, reducing drag. So don't you end up still getting the lift but it cost a bit more energy to get it up to lift off speed? But then you save energy at a displacement cruise speed ? Maybe I'm clinging to a bad idea on this one but it just seems, as you said,. like such a nice idear

I can see where there's an obvious trade off but who knows, just thought I'd ask. Thing is some folks seem to hate foils and some folks are stupid for foils and I'm always wondering if that doesn't taint some of the stuff I read about them. If you go to a foils site they act like its gods gift to the world. If you listen to most naval architects they all seem to hate them. But there must be some practical application for these things somewhere

[Ad Hoc]

Originally Posted by Boston

this might be a completely stupid question but the drag effects the power needed to generate the lift. But the lift still lifts and reduces the immersed area of the hull, reducing drag. So don't you end up still getting the lift but it cost a bit more energy to get it up to lift off speed?

It is all a mute point if you cannot manufacture a hydrofoil to take the loads. Hence theory into practice tends to shoot these great ideas down. One cannot cherry pick one aspect of "an idea" from theory to prove a point; which affects so many other disciplines that need to be satisfied or balanced in a design.. When the point is, you can't make it to begin with!

The foils have a limited applicable, and as you say, you either love them or hate them.

[Boston]

oh well, another one bites the dust